1. Field of the Invention
This invention generally relates to radio systems and, more specifically, to a precision timing generator for impulse radio technologies, such as communication systems, radar, and security systems.
2. Related Art
Recent advances in communications technology have enabled communication systems to provide ultra-wideband communication systems. Among the numerous benefits of ultra-wideband communication systems are increased channelization, resistance to jamming and low probability of detection.
The benefits of ultra-wideband systems have been demonstrated in part by an emerging, revolutionary ultra-wideband technology called impulse radio communications systems (hereinafter called impulse radio). Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990), U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) and U.S. Pat. No. 4,743,906 (issued May 10, 1988) all to Larry W. Fullerton. A second generation of impulse radio patents includes U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and co-pending Application Ser. No. 08/761,602 (filed Dec. 6, 1996; now allowed) to Fullerton et al. These patent documents are incorporated herein by reference.
Basic impulse radio transmitters emit short Gaussian monocycle pulses with tightly controlled pulse-to-pulse intervals. Impulse radio systems use pulse position modulation, which is a form of time modulation in which the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random (PN) code component. Generally, spread spectrum systems make use of PN codes to spread the information signal over a significantly wider band of frequencies. A spread spectrum receiver correlates these signals to retrieve the original information signal. Unlike spread spectrum systems, the PN code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain, and jamming resistance (interference rejection.)
Generally speaking, an impulse radio receiver is a homodyne receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Each data bit time position usually modulates many of the transmitted pulses. This yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. The cross correlator of the impulse radio receiver integrates multiple pulses to recover the transmitted information.
In an impulse radio communication system, information is typically modulated by pulse-position modulation. That is, the time at which each pulse is transmitted is varied slightly from the predetermined pulse-to-pulse interval time. One factor limiting the effectiveness of the communication channel is the accuracy with which the pulses can be positioned. More accurate positioning of pulses can allow the communication engineer to achieve enhanced utilization of the communication channel.
For radar position determination and motion sensors, including impulse radio radar systems, precise pulse positioning is crucial to achieving high accuracy and resolution. Limitations in resolution of existing systems are partially a result of the limitations in the ability to encode a transmitted signal with a precisely timed sequence. Therefore, enhancements to the precision with which timing signals can be produced can result in a higher-resolution position and motion sensing system.
Impulse radio communications and radar are but two examples of technologies that would benefit from a precise timing generator. A high-precision timing generator would also find application in any system where precise positioning of a timing signal is required.
Generating such high precision pulses, however, is quite difficult. In general, high precision time bases are needed to create pulses of short duration having tightly controlled pulse-to-pulse intervals. Currently available analog or digital integrated circuit timers are not capable of creating such high precision pulses. Typical impulse radio timer systems are relatively complex, expensive, board level devices that are difficult to produce. A small, low power, easily produced, timer device would enable many new impulse radio-based products and bring their advantages to the end users.
The present invention relates to a timing generator that provides highly accurate, stable, low jitter, and agile timing signals in response to a rapidly changing timing command input. Such signals are needed for UWB transceiver and radar devices as well as numerous other applications in industry and instrumentation.
Timing signals generated in accordance with this invention result in a signal transition at a precisely spaced (delayed) time relative to a time framing signal also generated by the system. The framing signal is typically slaved to a stable reference. In one embodiment, a phase locked loop (PLL) is used to accomplish this function. If the timing command meets certain setup time requirements, the output timing signal transition will be placed at a precise time relative to the associated frame signal transition. An early/late command input signal and associated mechanism are included to permit 100% time command coveragexe2x80x94free of gaps caused by setup time or metastable restrictions.
The invention utilizes a coarse timing generator and a fine timing generator to accomplish this goal. The coarse timing generator is utilized to define the framing interval and to further subdivide the framing interval into coarse timing intervals. The fine timing generator is used to define the time position between coarse timing intervals.
The coarse timing generator utilizes a high-speed synchronous counter, an input command latch and a digital comparator. One embodiment permits latching the input command at several points to permit 100% timing coverage. Another embodiment includes selectable counter lengths to scale the system to different frame rates and different reference timer frequencies. These setup parameters can be loaded using a serially loadable command register.
The fine delay generator is based on a phase shift circuit. Two example embodiments are described. One is based on a sine/cosine multiplier phase shift circuit; the other is based on an RLC switched element phase shift circuit. The sine/cosine multiplier circuit utilizes a sine wave version of the coarse delay clock together with analog voltages representing the sine and cosine of the desired phase shift angle to produce a sine wave timing signal shifted in time (phase) fractionally between two coarse delay intervals. In one embodiment, the fine timing generator uses an analog command input and as a result has a continuous rather than quantized transfer function. In another embodiment, the fine timing input is digital and is mapped through a memory device that drives a digital-to-analog converter (DAC) to produce the correct timing associated with the digital input command. This signal is combined with the coarse delay signal to produce the output delay signal, which is the sum of the two delays. In one embodiment, the delay generator contains two sets of sine/cosine generators to permit 100% timing coverage.
A unique advantage of the combiner circuit is that the coarse delay signal may have errors much larger than the final timing requirement. The coarse delay signal is only used to select among several fine delay signals. The fine delay signals determine the precision of the output.
An alternate embodiment of the phase shifter utilizes switched lumped element phase networks. This arrangement takes a direct digital input and does not need a DAC or a sine/cosine lookup table.
One embodiment of this invention implements the coarse and fine delay sections in a SiGe ASIC (chip) and partitions the system such that random access memories (RAM""s) and digital-to-analog converters (DAC""s) are external to the ASIC. In this embodiment, further advantages result from implementing the circuitry in fully differential current steering logic and differential analog amplifiers such that the chip draws a constant current independent of clock frequency. This minimizes on-chip transients that could introduce jitter in the output.
One of the unique challenges of UWB transceivers is that they not only need stable and accurately timed pulses, typically to 30 picoseconds (Ps) and stable over millisecond (ms) correlation intervals, but the timing needs to change dramatically from pulse to pulse. This interval may be on the order of 100 nanoseconds (ns) and needs to be accurate to fractional parts per thousand relative to an implementation-specific standard 100 ns frame interval. This invention has demonstrated the ability to meet these timing requirements and when implemented in ASIC form, it enables the production of relatively economical UWB systems.
For simplification, the invention is described by referring to diagrams that are single ended, but the preferred implementation is to use differential circuits. Various input and output signals are shown as differential.
An advantage of the invention is that timing pulses can be precisely positioned in time to a high degree of accuracy. As a result, advancements in communication technology can be realized. For example, in a communication system utilizing pulse position modulation, gains can be achieved in coding and bandwidth by taking advantage of the ability to more precisely position the timing of the pulses in the nominal period.
Additionally, because precise positioning can be achieved over the entire nominal period, the entire period can be utilized for communication, thereby resulting in increased channelization of the communication system.
Advantages are also realized in radar and motion sensor applications. More precise positioning of output pulses allows a higher resolution radar and motion sensor system.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying figures.